Course: Chemical Technology (Organic)
Module III
Lecture 3
Recovery of Chemicals from
Kraft Pulping Process
LECTURE 3
RECOVERY OF CHEMICALS FROM KRAFT
PULPING PROCESS
Chemical recovery system is an integral part of pulp and paper units and the efficiency of
chemical recovery plays an important role in economics. Efficiency of chemical recovery
units in terms of thermal efficiency, energy recovery and conversion of thermal energy to
electrical energy has tremendous impact on overall economy of pulp production
[Kulakarni, 2010]. Although earlier agro-based mills were not having recovery furnace
due to high silica content of black liquor and smaller capacity, however now chemical
recovery in agro-based mills have been made mandatory. Important terms in recovery of
chemicals are given in Table M-III 3.1.
Table M-III 3.1: Important terms in Recovery of Chemicals
Active alkali
Effective alkali
Total titratable alkal
Sulfidity
NaOH +Na2S
NaOH+1/2 Na2S
NaOH +Na2S+Na2Co3
Na2S/Titratable alkali
Causticty
Causticity Efficiency
Reduction
Over all recovery
NaOH/ NaOH +Na2S
[NaOH/ NaOH +Na2S )] x 100
Na2S/( Na2S04 + Na2S )
Recovery = black liquor stock/ fresh liquor consumed
KRAFT RECOVERY
Recovery of chemicals from black liquor is very important for overall economy of paper
industry. Kraft recovery process flow diagram is shown in Figure M-III 3.1. Various
sections in kraft recovery plant are
 Evaporation section and further concentration: Multiple effect evaporator, direct
contact, cascade evaporator, Cyclone evaporator, venture scrubber

Recovery furnace: Furnace, ESP [Figure M-III 3.2]

Dissolving Tank and Green liquor generation

Causticizing and clarification

Lime sludge reburning
114 Figure M-III 3.1: Kraft Recovery Process
Figure M-III 3.2: Kraft Recovery Furnace
115 Black Liquor Oxidation: Black liquor oxidation has become integral part of
recovery section for reducing the emission of volatile sulphur compounds. Oxidation of
black liquor by air reduces the emission of odorous sulphur compounds. Mecrcaptans and
other sulphur compounds are oxidized into less volatile chemicals. Both weak black
liquor and concentrated black liquor oxidation has been reported. Typical Composition of
Black Liquor from Wheat Straw Pulping is given in Table M-III 3.2.
Various steps involved in kraft recovery section are

Concentration of black liquor with inlet 12-18percent and outlet 40-50 percent
long tube evaporator. Normally five to six effect evaporator is used. As the black
liquor become highly viscous, further concentration is done by direct contact
evaporator and falling film evaporator

Further concentration in direct contact evaporator to 60%. Using cyclone
evaporator, cascade evaporator and venturi scrubber. In some of the mill Falling
film evaporator is being used to achieve higher concentration of black liquor
which is not possible with conventional evaporator. Other

The concentrated black liquor is sprayed in the furnace where black liquor is
incinerated to yield smelt and flue gases. A typical recovery furnace consists of
air preheater, economizer, furnace and boiler, cyclone, ventury scrubber/
electrostatic precipitator.

Addition of makeup chemicals Na2SO4 (salt cake) in thick concentrated black
liquor which is reduced to Na2Sin the furnace in presence of carbon

Incineration of Black Liquor in recovery furnace

Reducing zone in bottom to burn organic

Dying zone

Oxidation zone
Reactions in recovery of chemicals from black liquor
The main reactions are (1-3 predominantly)
Na2O+CO2→Na2CO3
Na+ + H+ + S2-→NaHS
116 Na + OH-→NaOH
Na2O+SO2+1/2 O2→Na2SO4
Na2S+2O2→Na2SO4
Na2SO4+Na2S+O2→2Na2S2O3
and other reactions:
Na2SO4+2C→Na2S+CO2
Na2SO4+4CO→Na2S+4CO2

Dissolving smelt in dilute wash liquor or water to produce green liquor.
Containing sodium carbonate and sodium sulphide.

Causticising of green liquor with lime to produce white liquor, lime sludge. A
typical causticising section consist of lime slaking with green liquor, dreg
clarifier, causticising tank, clarifiers, rotary filter, lime kiln.
Na2CO3+CaO +H2O

2NaOH+ CaCO3
Burning of lime sludge to produce lime.
CaCO3
CaO + CO2
Table M-III 3.2: Typical Composition of Black Liquor from Wheat
Straw Pulping
Parameters
Wheat straw Black liquor
Total solids
10.8
Swelling volume ratio ml/g
10
Gross calorific value
2890
Silica as SiO2% , w/w
2.87
Acid insoluble as SiO2 %, w/w
3.0
Chloride as Cl, %, w/w
4.3
Potassium as K, %w/w
6.0
Sodium as Na%, w/w
15.0
Calcium as Ca %, w/w
0.08
117 Inorganic as NaOH%, w/w
35.01
Organics%, w/w
64.99
Typical Characteristics of Black liquor
1 ton of black liquor - 2.2 ton of steam
Black liquor solid=1.30 ton /ton of pulp.
Weak black liquor per ton of pulp -10 -12 m3
Calorific value of black liquor: 3080 cal/gm
Black liquor-Organic 70%, Inorganic 30%
Weak black liquor: Na 28 gm/lit
RAA10.85 gm/lit
RECOVERY OF CHEMICALS IN AGRO-BASED MILL
With the shortage of forest based raw material and to preserve the forest, the paper
industry in Asia and other parts of the world is forced to use non woody fibrous raw
materials. India is one of the leading countries using substantial proportion of non wood
materials which constitute about 50% of total raw material furnish [Dixit et al., 2012].
Non-wood raw materials are characterized by high silica content which varies from 1.5 to
20%. Most of the silica present in raw material passes into black liquor. Presence of silica
in relatively high concentration creates numerous problems in chemical recovery plants.
[Dixit et al., 2010]. Evaporator, recovery furnace, and lime kiln operation are adversely
affected by presence of silica in black liquor [Ibrahim,1988].The process of distillation of
bamboo and straw black liquors by carbonation has been developed by central Pulp and
paper Research institute, Saharanpur,
Due to high silica content of the agricultural residues recovery of chemical from agrobased mills have caused major concern in Indian paper mills. Table M-III 3.3 gives the
details of processes for recovery of chemicals from agricultural residues black liquor.
Black liquor from cooking agricultural residues has high silica content and cannot be
concentrated to high solid contents to enable its burning in conventional recovery
118 systems. Although, technologies are available and being practiced in many mills in other
part of world; however, due to poor infrastructure still recovery of chemicals is not being
practiced by majority of mills in Indian posing serious environmental pollution problems.
Pollution load in agro-based small mills having no chemical recovery is almost three
times to that of pollution load in large integrated mills
Some of the currently accepted technologies for recovery of chemicals for agro-based
black liquor are: Rotary roaster, Roaster cum smelter, Broby smelter, Dry pyrolysis
Process, Wet air oxidation process, Copeland reactor. Some of the emerging technologies
are Wet cracking process, Chemsec process, Thermo chemical conversion reactor system,
Super critical wet oxidation process, Direct causticization process, ABC process, Direct
alkali recovery system (DARS) or Ferrite process. In the fluidized bed soda recovery
system where the weak black liquor containing high silica content is concentrated to
about 45percent concentration in multiple effect evaporator, venturi scrubber and cyclone
system and is sprayed in the fluidized bed combustion system.
Table M-III 3.3: Processes for Recovery of Chemicals from Agricultural
Residues Black liquor
Process
Description
Ferrite process (DARS 30% conc. Black liquor burnt with ferric oxide at temp 850 0c.
process)
Wet air oxidation process
Na2CO3 +Fe2O3
2NaFeO3 +CO2
2NaFeO3 +H2O
2NaOH + Fe2O3
Oxidation of organic matter at high temp 320-320 0c pressure
175-250 kg/cm2.
Ultra
Filtration
and Sodium compound is removed as permeate. High molecules
reverse osmosis
weight lignin fraction is rejected in interface.
COPE LAND process
34-35% Solid liquor is burnt in fluidized bed reactor. 650-
119 700oC.
Evaporation 35-37% in evaporator. Last two effect falling
BKMI Process
film. Upto 50% in direct contact evaporator, then concentrated
black liquor is burnt.
cyclone 18-20% evaporation in multiple effect evaporator upto 50%
Smelter
inn cyclone evaporators. Concentrated black liquor is fired in
evaporator
furnace.
Destomax
recovery Similar to Smelter cyclone process
system
Burning of Black liquor Removal of silica by carbonation followed by concentration
after desalination
and burning
References:
1.
Dixit, A.K., Jain, R.K., Mathur, R.M., “Incineration of black liquor mixed with
waste biomass-An option to achieve zero black liquor discharge in small agro based
paper mills” volume 245, No.3, 2012, p.161
2.
Dixit, A.K., Thapliyal, B.P.,, Jain, R.K., Mathur, R.M., “Desilication of Bamboo &
straw black liquor” IPPTA, Vol 22, No.3, 2010, p.121
3.
Ibrahim, H. “Proceedings of the International non-wood pulping and paper making”
Beijing, Vol II, 1988 p.877.
4.
Kulkarni, A.G. “Removal of non-process elements in chemical recovery-A review
technologies” Ippta, Vol. 22,No3.3, 2010, p.113
120